No connection to mysql database from other subnet

I`ve installed a ISPConfig3 multi server setup with a webserver, a mailserver and a dns-server.
Then i installed a second dns-server in an other remote-subnet.
My problem is, that i can't connect it to my database, which is located on the web server.
When configuring the second dns-server, this question appears and i answer with "yes":

Shall this server join an existing ISPConfig multiserver setup (y,n)

I entered the correct master-server host, user and password, but the answer is always:

Unable to connect to mysql server

Although I'm working as root, i don't have permission to see/modify mysql.user

1) Check that you opened the mysql port in the firewall on the master server.
2) Ensure that you added root users in the mysql server on the master for all IP addresses of the slave server that shall be used to connect. If you are unsure about the IP addresses, use % instaed of the IP when creating the mysql root user.

I tested to connect to other sql server and i was able to connect to all of them. Some of them are in the same subnet, some are not.
But connecting to the master-server isn`t allowed to any of my servers.
By using "telnet" i always get this message:

Could it be that there are other security settings except username and password?

Click to expand...

Mysql uses a security concept which takes the IP address in account as I explained above. If you want to test the mysql connection, use the mysql command and for the test you have to run it on the salve and not on the amster or any other server.

So to test the connection, login on the shell of the slave server and issue this command:

mysql -h master.server.tld -u root -p

replace master.server.tld with the hostname of your master server.

Another important thing is that you edited the /etc/hosts files eaxctly as described in the installation instructions on all servers that belong to the clusetr. E.g.